Cleaning porous media



Nov. ll, 1952 C, R, STEWART f 2,617,719

CLEANING PoRoUs MEDIA Filednec. 29, 195o INVENTOR. CHARLES R.STEWART A T TORNE Y "with uniform porosity and of small size.

ever, inlimestone or dolomite cores, which simul- Patented Nov. 11, i1952 UNITED CLEANING POROUS MEDIA Charles R. Stewart, Tulsa, Okla., assigner to Stanolind Oil and Gas Company, Tulsa, Okla., l a corporation of Delaware Application December 29, 1950, Serial No. 203,3584

14 Claims.

This invention relates to the analysis of porous media Vsuch as the mineral cores recoveredin drilling wells for production of fluids such as oil and gas, and is directed more particularly to a process for cleaning from the pore spaces within such media the soluble substances found therein prior to making measurements for determining porosity and permeability.

Careful study has shown that measurementsof porosityand permeability on limestone and dolomite cores, in which a major portion of the permeability is due to cracks, fissures, and solution cavities, must be performed either on fairly large-sized core samples or on a large number of small samples before reliable data are obtained. The variations in porosity and permeability from point to Doint'inl such cores areso great that small-core measurements, unless averaged in large groups, are virtually meaningless, as the fluid passages are often of comparable magnitude with the small cores themselves. Even in measuring the permeability and porosity ofsandstonecores which exhibit a fairly uniform type of porosity, the first step is a thorough cleaning of the core sample to remove all naturally occurring or invading soluble substances, leaving only the rock material with air or gas filling the pore" spaces. Then the percentage Volume (porosity) and the interconnection (permeability) of the pore spaces can be properly measured.

The procedure heretofore used for cleaning cores has generally been the Soxhlet process, or some modification thereof, in which hot solvent passes downwardly over and through the core sample by gravity drainage and, over a considerable length of time, leaches out the soluble materials from thepore spaces within the core. Although the Soxhlet process is somewhat timeconsuming, it has been fairly effective for cores Howtaneously exhibit two types of porosity-a low permeability in the rock matrix itself, coupled with a higher permeability in the form of cracks, fissures, `and solution cavities, particularly when such cores are of substantial size-the hot solvent of the Soxhlet apparatus tends to flow only 4through the major fracture or solution channels and bypasses the very much smaller pores and flow channels of the rock matrix itself. In actual tests of the Soxhlet process on some large cores of the limestone and dolomite type of porosity, a Soxhlet extraction was carried out for periods as long as six weeks at atime; but, at the end of that period, the cores were still stained with.

oil, indicating incomplete and ineffectiv'e'cleaning. As a matter of fact, prior to the present invention, neither the Soxhlet extraction process 'nor any of its generally known modifications has been able, in a reasonable length of time, to clean large-sized limestone or dolomite cores, for example, up to 5 inches in diameter and 2 feet in length, so that reliable porosity and permeability determinations could bemade. Also, on small cores and Vvplugs having very low permeability,

Vthe Soxhlet process is either ineffective or requires unreasonably long periods of time.

' It is accordingly a primary object of my invention to provide an improved process for cleaning porous media such as rock'cores, regardless ofthe typeof porosity. Further and more ispecic objects, stated briey, are to provide a core'- cleaning process which is rapid, highly eiective, inexpensive, and adaptable to cores ofany size, with permeability which may either be uniform or include cracks, fissures, and solution cavities.

The foregoing and other objects are accomplished by a process which comprises generally the steps of first forcing into the pore spaces of the medium to be cleaned a liquid solvent `containing a dissolved gas phase at an elevated pressure, preferably at an elevated temperature, and then rapidly lowering the pressure on the liquid solvent and the porous medium below the bubble-- point of the gas dissolved in the solvent, whereby the gas comesv out of solution within the pore spaces of the medium and drives out the liquid solvent, together with dissolved substances. This sequence of steps is repeated a number of times, depending upon the character of the medium, until all parts of the medium are` completely clean. l l 4 l This will be more readily understood by reference to the accompanying drawing'forminga part of this application, which drawingshows, partially diagrammatically and partially in4 cross section, one embodiment of an apparatus for carrying out this process.

The apparatus shown is particularly suitable for carrying `out the cleaning process ofthe invention on cores recovered, from oil and 'gaswells The chamber in which the cores'are placedjfor cleaning is a cylindrical cell l0 closed at one `end and provided with a removable closure for the other end comprising a threaded cap Il which holds in place within the bore of the cella sealing member I2 carrying an O-ring seall I3. v- Cell chamber I0 is preferably inclined at a few degrecs fromthe horizontaland ,is surrounded,v at least partially, by a steam chamber 15 connected through a valve I6 to a steam line |1 and provided with a drain I8. A uid duct 20 enters the bottom of cell chamber l at the lower end, and a similar fluid duct 2| passes through the top wall of the cell I0 at the upper end.

To the lower duct 2|) is connected a conduit 23, which leads through a valve 3|) to the bottom of a saturating drum 3| containing a gassaturate liquid solvent 32 at an elevated pressure. In the upper or inlet end of drum 3| are a number of horizontal screens or perforated bales 33 underneath a spray nozzle 34. From a perforated pipe 22 extending across the lower part of drum 3| within solvent 32 a conduit 25 extends through a valve 24 to the low-pressure side of a gas-pressure regulator 26 connected to a cylinder 21 or like supply of high-pressure gas. The conduit 23 is additionally connected through a valve 35 to the outlet of a hydraulic pump 36 which has its inlet connected to the bottom of the saturating drum 3|.

Extending from the upper iiu-id duct 2| is a conduit di! leading to a valve and thence through a conduitY 42 to the regulator 26. Connected also to the conduit i0 is a pressure gage 43, by which the pressure within cell I0 is continuously indicated. From the conduit 23 adjacent duct a conduit H- controlled by avalve t5 .extends to a vessel 45 for storing used solvent until it can be reclaimed. Also leading into vessel 46 from the conduit @El is a conduit 38 controlled by a valve 39.

Located preferably adjacent or abovethe drum 3| is a vessel 41. for storage of clean solvent, the bottom of which vessel is connected through a valve 48 to nozzle 34 in the top of drum 3|. From the top of vessel 1 a conduit 50, `including a valve 5|, extends to the outlet of a regulator valve 52 which is connected with the high-pressure gas supply cylinder 21.

The operation of this apparatus is Aas follows: With all valves initially closed, a quantity of gassaturated solvent is prepared by first opening the regulator valve 26 and valve 24 to provide within the saturating drum 3| an elevated gas pressure, for example, 200 p. s. i. Then regulator i52 is set to some higher value,- say p. s. i., so that, upon the opening of valves 5| and ib-clean solvent from vessel 41 issprayed through nozzle '34 and over screens 33 into drum 3|. In the presence of the Y gas previously or concurrently introduced into the drum 3|, this solvent becomes substantially gasfsaturated at the down-stream pressure of regulator 26. When a sufficient quantity of liquid solvent 32 in drum 3| has been saturated for at least one filling of cell I', valves 5| and 48 are again closed.

After one or more cores 55 to be cleaned have been placed in the cell |0 and sealed therein by the cap and seal ring I3,- the cell lll is first filled with gas from cylinder 21 through regulator 26, set at '200 p. s. i., by opening valve lll. Gas from regulator 26 then ows through conduitsl 42 and 40 into cell I0 until it and the Vcores; therein are charged with gas at the downstream pressure of the regulator 2s. Thereafter, the valve 4l is closed, and valve is opened. Then the gas pressure within cell l0 is bled oif slowly through the valve 39 so that solvent 32 flows through conduitv23 and into the bottom of cell ||l. As soon as vessel IU is completely lled with liquid solvent, all of the gas having been driven out, valves 30 and 39 are closed, thus completely shutting olf the cylinder I0 from the rest'of the system. The pressure is then raised on the liquid within cell |0 to force it into the pores of the cores 55. This is done in either of, or a combination of, two ways. The temperature of the solvent and of the cores within the vessel I0 is raised to assist in the solvent action by opening valve l in steam line I1 to admit steam into the chamber I5. In many instances, due to thermal expansion, the step of merely increasing the temperature of the liquid solvent containing the dissolved gas is alone sufficient to increase the fiuid pressure by several hundred per cent, as indicated by the gage 53. The reading of gage d3 is watched closely, and the valve 39 is manipulated, if necessary, to bleed off a small amount of solvent to prevent the pressure from risingr to values in excess of that which the cylinder It can withstand, for example, 1,000 p. s. i. In the event the pressure rise, due to heating by the steam chamber I5, is insucient to drive the gas-containing solvent into the pore spaces of the cores 55 as desired, additional pressure is supplied by opening the valve 35 and forcing additional liquid solvent into the vessel l0 by pump 36. After a period of time, of the order of 15 minutes, when it is noted from gage i3 that the high pressure remains constant, and also after allowing a time interval, after the solvent has penetrated the pore spaces, for the dissolving action to take place under the increased temperature, the valve 39 is opened, allowing solvent and gas to pass into vessel E so that the pressure in cell I9 drops rapidly to a low value. This rapid reduction in pressure below the bubble point of the gas in the solvent 32 results in the formation of free gas, both within the vessel l0 in the space around cores 55and within the pore spaces of the cores themselves. As a result, the solvent liquid within the pore spaces, which has contacted and dissolved at least part ofv the soluble substances within the pores, is rapidly driven out by the expanding gas bubbles, carrying the dissolved substances. During this driving period, the cores 55 are preferably kept surrounded by liquid by limiting the flow through valve 39, so that, as the waxor oil-laden solvent emerges from the cores, it mingles with relatively clean solvent surround ing the cores. Then, when the gas drive has exhausted itself, any of the solvent which may be drawn back into the cores 55 by the action of capillary forces will be relatively dilute rather than heavily laden with wax and oil. At the end of the gas driving of the solvent from the cores, the remainder of the liquid within chamber l0 is withdrawn through valve 45 and line 44 to the used solvent storage 45, and the cycle is repeated by again filling the chamber l0 with gas from the cylinder 21 through line 42 and valve 4|. This series of steps of ooding the cores with gassaturated liquid, forcing it into the pore spaces, and suddenly releasing the pressure, is repeated for a number of times, varying as a rule between three and twenty, depending upon the character of the core material being cleaned, until substantially all of the soluble substances within the cores have been dissolved and removed.

The liquid solvent used may be any of the conventional. solvents used for dissolving the substances found in cores: for example, toluene, benzene, xylene, carbon disulfide, a halogenated hydrocarbon, or the like. The gas to be dissolved in the solvent liquid is preferably one which has a substantial solubility in that liquid. Carbon dioxide is quite suita-ble for many cases, although nitrogen or other relatively inert gases might also be used. An advantage of the mixture of carbon dioxide and toluene, which has been `most frestances within the pore spaces. `is that the'pressure used to force the gas-satlowerlimit, of course, being '5 'quently used, 'is that ,the solubility of the gaseous carbon dioxide in the toluene is nearly 100 times thesolubility of nitrogen, for example, in toluene. A suitablepressure setting for` regulator 26 when using carbon dioxide isaboutv 20G-p. s.vi., and the pressure on the confined liquid solvent in the core chamber, as read from the gage 43,l is normally held between 600 and 1,000 p. s. i. during the period of time' rthat the solvent is being driven linto andheld in the pore spaces ofthe cores, heat is beingappliedby steam chamberV l5, and

the solvent is being allowed to act on the sub- A general rule urated liquid solvent into` the core is about five 'times' the bubble-point pressure of the gas-solvent mixture, although thisis notfat. all critical and higher or lower pressures may be-used, the 1 the bubble-point pressure of thesolvent.

It is also possible to utilize in the present process a single solvent having a fairlyV high vapor pressure at thev temperature to which the core and solvent is raised. For example, propane or butane, or a mixture of propane, bultane, 'and ethane in the proper proportions would have a suitable vapor pressure to create a` satisfactory gas drive after being forced at high pressure into the pore spaces of the cores 55. Likewise, a similar extraction might be carried out using a heavier hydrocarbon solvent such as pentane or hexane containing methane or natural gasy dissolved therein as the dissolved-gas driving me- The foregoing represents only one of several possible ways of applying the principles of my invention to core cleaning. Somewhat less convenient but equally effective is rthe step of introducing the gas-saturated solvent by circulating it through the core from end to end or side to side. Upon then suddenly reducing the pressure below the bubble point, as gas drive takes place within the pore space as described.

Another procedure is to fill the pore space rst with gas of high solubility in the solvent and then force an under-saturated liquid into or through the core at high pressure, so that the gas goes into solution within the pores themselves. Pressure reduction then establishes a gas drive as described, but it is believed to be Somewhat less eiective than in the method described in detail because the dissolved gas distribution within the pore space is less uniform.

While I have described my invention with reference to the foregoing specific details and examples, it is to be understood that still further variations and modications will be apparent to those skilled in the art. The invention, therefore, should not be considered as limited to the described details and examples but is to be ascertained from the scope of the appended claims.

I claim:

l. The method of removing soluble substances from the porcs of a porous earth sample which comprises the steps offorcing into the pores of said sample at elevated pressure a liquid solvent containing a dissolved gaseous phase, and thereafter rapidly reducing said elevated pressure below the bubble point of said gaseous phase, whereby free gas is formed within said pores to drive said liquid solvent carrying said substances through said sample.

2. The method of removing soluble substances from the pores vof a porous earth sample which Vcomprises saturating a liquid solvent for said substances with a gasV soluble `therein at elevated pressure, forcing said solvent containing said gas dissolved therein into said pores at 'ahgh pressure at least as high as said elevated pressure, and thereafter rapidly reducing the pressure on saids'ample and solventbelow the bubble point of said gas in `said liquid solvent, whereby free gas isV formed Awithin said -sample and' expands to drive said solvent and dissolved substances through said sample. Y

3. 'The' method of cleaning liquids and soluble solids from earthcores comprising dissolving in a liquid fsolvent `a gas atelevated pressure,` forcing s'aidfliquid solvent retaining said gas in solutionintoa core while maintaining said pressure, andthereaflter reducing saidpressure to a low value, whereby said gasA comes out of solution within the pore spaces of said core and thereby forces 'said solventout of said core carrying'with it at least a portion offsaid liquids and soluble solids.i

4, The methodof cleaning liquids and soluble solids from earth cores comprising dissolving a gas in a liquid solvent at elevated pressure, circulating said solvent containing said gas in dissolved state through said core while maintaining said pressure, thereafter reducing'the pressure on said core and solvent to a low value, whereby said gas comes out of solution within the pore spaces of said core and expels said solvent carrying dissolved liquids and solids from the interior `of said core, and repeating said circulating and pressure-reducing steps. I

5. 4The method of removing soluble ,constituents from within the pores-of a porous" earth sample which comprises the step of circulating through said sample a`solvent for said constituents containing a dissolved gas held in solution by pressure, and the step of reducing the pressure surrounding said sample to cause said gas to come out of solution within said pores and thereby expel said solvent carrying said constituents, and repeating said steps in sequence until said constituents have been substantially completely removed.

6. The method of removing soluble substances from the pores of a porous earth sample which comprises forcing into said pores aft an elevated Y pressure a liquid solvent containing a dissolved gaseous phase at said elevated pressure, increasing the temperature of said liquid and said sample to promote the action of said solvent on said substance, and thereafter reducing said elevatedl pressure on saidsample and liquid to a value below the bubble point of said gaseous phase, whereby free gas is formed Within said pores to drive said liquidY solvent carrying said substances through said sample.

7. The method of removing soluble substances from the pores of a porous earth sample which comprises establishing within the pores of said sample a'mixture of a liquid solvent for said substances and a driving gas while maintaining a pressure on said sample above the bubble-point pressure of said gas in said solvent, and thereafter rapidly reducing said pressure to a value below said bubble-point pressure, whereby free gas is formed in said pores to, expel said solvent carrying dissolved substances from said sample.

8. The method of cleaning liquids and soluble solids from earth -co-res which comprises enclosing a core within a closable container, filling the space in -said container around said core with a liquid solvent containing a dissolved gas at elevated pressure, raising Ithe temperature of said container and its contents to increase the fluid pressure therein due to thermal expansion, whereby some loi. said solvent is forced into said core, and thereafter rapidly reducing said pressure `below the bubble point of said gas, whereby free gas is formed within said core to drive out said solvent carrying dissolved substances.

9. The method of cleaning liquids and soluble solids from earth cores which comprises enclosing a core within a closable container, filling said container with gas at an elevated pressure, displacing said gas from the space surrounding the core in said container with a liquid solvent saturated with said gas at said elevated pressure, increasing the pressure on said liquid solvent in said container substantially above said elevated pressure to force some of said gas-saturated solvent into said core, and thereafter rapidly reducing the .pressure within said container below the bubble point of said gas in said solvent, whereby free gas is formed within said core to drive out said solvent carrying dis-solved substances.

10. The method according to claim 9 in which the pressure on said liquid solvent in said container is increased to about ve times said elevated pressure.

1l. The method according to claim 9 including also the step of increasing the tempera-ture of said container and its contents while the pressure on the solvent inV said container is above said elevated pressure, whereby the action of said solvent is promoted.

12. The method according to claim 9 in which said gas is carbon dioxide, and said elevated pressure is about v200 p. s. i.

13. The method of cleaning earth cores which comprises the steps of enclosing a core within a closa/ble container, lling said container with carbon dioxide gas at about 200 p. s. i., displacing said gas with toluene saturated with carbon dioxide at about 200 p. s. i., increasing the pressure within said container to about 1,000 p. s. i., whereby said saturated toluene is forced into the pore spaces yof said core, heating said container and its contents substantially above the surrounding ambient temperature, rapidly reducing the pressure within said container substantially to atmospheric, whereby said carbon dioxide comes out of solution within the pore spaces of said core yand expels said toluene carrying dissolved substances therefrom, and withdrawing said toluene yand dissolved substances from said container.

14. The method according to claim 13 in which all of said steps except the first are repeated in sequence for a number of times between three and twenty.

CHARLES R. STEWART.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,249,232 Squires Dec. 4, 1917 2,135,319 Bays 1 Nov. l, 1938 2,173,842 Horner Sept. 26, 1939 2,265,923 Normand Dec. 9, 1941 

1. THE METHOD OF REMOVING SOLUBLE SUBSTANCES FROM THE PORES OF A POROUS EARTH SAMPLE WHICH COMPRISES THE STEPS OF FORCING INTO THE PORES OF SAID SAMPLE AT ELEVATED PRESSURE A LIQUID SOLVENT CONTAINING A DISSOLVED GASEOUS PHASE, AND THEREAFTER RAPIDLY REDUCING SAID ELEVATED PRESSURE BELOW THE BUBBLE POINT OF SAID GASEOUS PHASE, WHEREBY FREE GAS IS FORMED WITHIN SAID PORES TO DRIVE SAID LIQUID SOLVENT CARRYING SAID SUBSTANCES THROUGH SAID SAMPLE. 